1. Field of the Invention:
This invention relates in general to drilling rigs, and in particular to a mechanism for absorbing reactive torque from a power drive unit of a top drive drilling rig.
2. Description of the Prior Art:
In recent years, top drive drilling rigs have been introduced. In this type of drilling rig, the power to rotate the drill string is supplied by a drive unit in the derrick. The drive unit is supported by the blocks and includes an electrical motor. When the drive unit reaches the rig floor, three joints of drill pipe secured together can be connected between the upper end of the drill string and the drive unit. This speeds up the drilling process over the prior type that applied rotation at a rotary table on the rig floor.
As the power drive unit rotates the drill stem, a reaction torque will be imposed on the case or housing of the drive unit. The reaction torque will tend to cause the housing to rotate in a reverse direction to the drill stem. To accommodate this reaction torque and stabilize the drive unit, a pair of vertical braces or guide tracks are mounted in the derrick. A pair of rigid stabilizing arms are rigidly connected to the case. Each stabilizing arm will engage and slide on one of the guide tracks.
One disadvantage is that when converting a conventional rotary table driven drilling rig to a top drive drilling rig, extensive modifications are needed to the derrick. The guide tracks must be installed. Sometimes there is insufficient room to handle the top drive, support carriage, guide tracks and hang off mechanisms within the upper derrick structure. Also, the additional forces imparted to the derrick structure from the reaction torque are not wanted.
Another type of top drive drilling rig uses a drive shaft that extends vertically into the derrick. The drive shaft is driven at the rig floor. A carriage is supported by the blocks. The carriage has a sprocket that slidably receives the drive shaft and rotates with the drive shaft. The carriage also has a sprocket that rotates a drive stem for connection to the drill pipe. A chain extends around the sprockets to transmit the rotary force from the drive shaft to the drill stem.
This type of top drive unit does not have reactive torque imposed on the carriage. Consequently, it does not need vertical guide tracks. It does, however, require a drive transmission at the rig floor to drive the drive shaft.